fix(chrome-ext): preserve cursor position in generic contenteditable replacement

[draphy]

Issues

Fixes #2616

Description

replaceGenericContentEditable used el.textContent = value, which destroys DOM structure and loses cursor position. This affects editors like Notion that fall through to the generic handler.

Now tries surgical replacement via selectSpanInEditor + replaceTextInRange first, falling back to the original behavior if that fails.

Also extracts testBasicSuggestionRichText and testCanIgnoreRichTextSuggestion shared test utilities (matching the existing testMultipleSuggestionsAndUndo pattern) and adds cursor position verification to the basic suggestion test.

How Has This Been Tested?

The basic suggestion test now verifies cursor position by using ArrowRight×3 + Backspace from the expected cursor position.

Checklist

• I have performed a self-review of my own code
• I have added tests to cover my changes